SHARC
Submillimeter High Angular Resolution Camera

Caltech Submillimeter Observatory

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First Night Procedures


These are a set of procedures that you should do only once, at the begininng of the run. Omitting to do these will not cause disaster, only a somewhat compromised efficiency... After you have followed through the steps of Beginning of Night procedures (for every night), you'll need to do the following (if possible).
 
 

Before You Start

You will have to be pointed and observe a reasonably bright source (preferably planets) for all the procedures below. If you cannot see the source with a single OTF pass (~ 5 arcsec/sec scan speed), it is probably pointless to go through with the phasing and flat-fielding procedures. Focusing can be performed with somewhat fainter sources as well, if chopping mode is used. If you need help with pointing click here.

 
1. Phasing

The phases are specific to chopper frequency. Therefore this needs to be done every time you change the frequency you chop at. The phases tell SHARC how much after the chopper control signal (TTL) transition should the bolometer be read out. Thus having incorrect phases will result in unoptimal readouts, i.e. loss of efficiency.

Use pixel 12 as your reference pixel. Do this by typing SHARC /REF=12 in the UIP prompt. (This is necessary as the phasing script assumes that you are pointed on pixel 12). After phasing and flat-fielding, feel free to chose any other pixel (we recommend 21) as your reference.

Make sure you are pointed, and observing a bright source (planets, M42.1, G34.3, IRAS16293, SGRB2 etc.).

To phase, run 'phase_12' from the UIP prompt. Then watch on the Mac as the various pixel phases appear. Apart from the bad pixels (1, 5, 15, 16) all should be within a few degrees of each other (in the ballpark of 260-280 degrees). If unsure, repeat the phasing until confident that a nice looking set of phases is obtained. When done, type 'sharc /save' to save the phases to disk.
 

2. Flat-fielding (if possible)

Flat-fielding is used to determine relative pixel gains. This is done by moving a bright point source accross the array. Practically only Venus, Mars and Jupiter and Saturn are really suitable for this purpose (also a few very bright Galactic sources, like G34.3) and only at reasonable elevations is it at all meaningful (between 10-60 degrees of Zenith). Moons of Jupiter might also work well, if you manually enter coordinates for them. Do not dispair if cannot do it at the beginning of the night. Doing it at any point during ther run is useful, as long as you have a set of measurements. Also, omitting it altogether isn't disastrous, as you can use gains from another recent run, with good confidence. Remember to flatfield after you adjusted the phases, as changing phase settings will almost certainly change your apparent pixel gain.To flatfield:
 
i. Point (see Pointing Procedures) with pixel 12 being your reference pixel.
ii. type 'ff_sharc' in the UIP to automatically flatfield.


The program will place the beam in both the 'on' and 'off' beams of the chopper, and move it accross all pixels. Thus two entries (on and off beam) are written to your output file, which you can look at by choosing 'gain' mode in 'camera'. Camera will also generate gains_1.dat and gains_2.dat, which you can use to calibrate.
Note. These gain corrections are only really needed when mapping. For point sources (when source is in 1 pixel really) not knowing realtive pixel gains is not really an impediment.
 

3. Focusing (if desired)

Focusing can only be performed efficiently on the bright point-like planets (all the way to Uranus, except Jupiter) at reasonable elevations (10-60 degrees of Zenith). Also, the moons of Jupiter perform well. In good weather, one can have some success also with the stronger point-like continuum sources if chopping (IRC10216, IRAS16293, G34.3 etc. can be used). If mapping, one should aim to get the smallest and most circular beam. When chopping, one can instead try to find the focus offset by the optimal signal. However, before one would sweat over doing this, it should be mentioned that there it is porbably OK to use the last offset (0.15 in January 2000) with confidence. Experience shows that focus is perfectly good in a wide range of settings (+=0.20), and the suggested setting falls in the middle of that range as last measured. There is little reason to believe the focus had changed unless SHARC or the telescope went through some major changes. Also bad focus results in no significant decrease in flux (for point sources), and thus it is only recommended to focus when obtaining ideal mapping resolution is desired, or if beam shape concerns arise.
To focus, you'll have to try manually different settings of focus offsets. To do that, type 'foc /off=X.XX' in UIP where X.XX is the offset you want to set (0.15 is recommended start point). See at which setting you get ideal beam (smallest most circular, or highest peak flux).

Last Updated 17 Sep 2001, by Attila Kovacs, attila@socrates.caltech.edu